I recently built a working system using a D410PT mini-ITX motherboard and 4GB of ram. Put everything in a mini-ITX case with 300w p/s. Computer works just fine using the on-board video.
I purchased a 512MB "EVGA" brand GeForce 8400gs PCI (not PCIe) video card for it because I wanted something faster than the on-board video (with the added benefits of using less cpu and none of my ram).
But it does not work. Card tested in another PC and works fine. Tried another REALLY old (Voodoo2) PCI video card in the D410PT, and that card worked as well, so it's not the slot. So why doesn't my motherboard detect the 8400?
D410PT mITX motherboard.
4gb of Kingston DDR2-800 ram.
ARK mITX case with 300w p/s (but I tried a 385w p/s and the card still didn't work).